What’s wrong with holding power button few (2-3) seconds more? Without taking battery out, until green LED light comes on.
Also, didn’t you try this advice ending with: “There should be no need to remove the battery to perform this sequence of steps.” Might easily serve the purpose of recovering the battery max. capacity (but not sure, I’m no expert)?
“What’s wrong with holding [the] power button?” Probably nothing, if that worked. This is not my first rodeo. None of the other steps worked (particularly the ones with the battery left in).
There’s a problem with the phone (and apparently the laptops) that goes beyond the users involved.
Yes, I’m not accustomed to the [the] usage when needed to be there (not even with sudo), thanks for this justified correction! My posts are more about substance, less about grammatical issues that I bring with my writing here, sorry for that (although important to be learned well, wasn’t focused either).
Back to your issue (let my think loudly 
), there is hardware (Librem 5) and there is BPP-L503, both products under warranty, yet I’d start with the brand new software (by erasing your whole/current eMMC setup … as only professional maintainers of particular Linux distribution doesn’t need this procedure, I guess), start with the brand new installation of PureOS Byzantium (Phone version, to include execution of: ./scripts/librem5r4-flash-image --variant A or B) that brings within it brand new open source firmware: git clone https://source.puri.sm/Librem5/librem5-flash-image.git (here is link to my earlier post) for your device.
Just command: sudo systemctl restart upower might be good starting (very first step) point (rather partial, please observe device’s battery percentage when executing it) to move on/forward from where you are locked now (presuming your Librem 5 is just “normal”, healthy device).
Please order another/replacement battery (if/perhaps already delivered one problematic). Even charging once or occasionally your battery outside of Librem 5 might help (beside other Forum tips) too, but not something to rely upon. Librem 5 isn’t polished product (yet) and won’t be (as personal usage device, up to every specific and/or “my own taste”), near to 100% of your or my liking (probably), but every individual that have interest into is able to get there (close to 100% of his/her personal satisfaction) even practically (if/when related to end-user controlled perspective). Translation: your post “request” is minimal/justified/expected indeed, IMO.
Besides, as I’m trying to make it short here: are you able to establish stable, data related, connection over Jumpdrive package (by using USB-A to USB-C cable, link to the Jumpdrive 0.8 image is here) with your Librem 5? This procedure involves some other kind of turning on, powering into your Linux phone, asking therefore (even way before of doing any actual kind of brand new installation).
P.S. This post doesn’t provide any kind of not-known solution, not even close to it, anyhow not without your progressive involvement (rather than extending your expectations toward Purism side, as I do think that they already produced, for sure 
, or at least worked hard toward even better solution), if so at all, yet it isn’t the one (this post), either, that explains on why you might be about to sell (potentially, within some other thread) this particular Linux phone of yours. I’d like to help (if/when enough time available), that’s all (please rethink your next move, how to upgrade this current state, as “no-other-possible-way” turning on procedure, still bothered with this particular problem/issue, since September last year), maybe your “factory” installed image caused/causing this related problem (who knows?).
Please test on/off (either right away or after few hours, actually both) with: sudo shutdown hF now. Also please execute the same command (after conecting device to its power supply) and charge your Librem 5 until red LED light off (if so?), disconnect, please, plugged-in power cable and (try to) turn it on to “note” if 100% there.